Moritz Stolpe

X-Ray Photon Correlation Spectroscopy Reveals Intermittent Aging Dynamics in a Metallic Glass.

We use coherent x rays to probe the aging dynamics of a metallic glass directly on the atomic level. Contrary to the common assumption of a steady slowing down of the dynamics usually observed in macroscopic studies, we show that the structural relaxation processes underlying aging in this metallic glass are intermittent and highly heterogeneous at the atomic scale. Moreover, physical aging is triggered by cooperative atomic rearrangements, driven by the relaxation of internal stresses. The rich diversity of this behavior reflects a complex energy landscape, giving rise to a unique type of glassy-state dynamics.

Electrochemical dissolution behaviour of Ti90Al6V4 and Ti60Al40 used for ECM applications

The dissolution behaviour of two different titanium alloys in aqueous electrolytes has been investigated. Therefore, a commercial titanium grade 5 (Ti90Al6V4) alloy and a self-produced Ti60Al40 alloy were compared. After preparation by arc melting and a heat treatment, an extensive characterization of the alloy by X-ray diffraction and energy-dispersive X-ray spectroscopy has been performed. The electrochemical behaviour of the alloys in different electrolytes was investigated using different techniques like linear sweep voltammetry and electrochemical impedance spectroscopy. An influence of the electrolyte composition and the titanium content of the alloy on the dissolution process could be observed. Higher titanium content of the alloy impedes the dissolution process. An increase of chloride ions in the electrolyte facilitates the dissolution. The results could be proved by an electrochemical machining (ECM) process in lab scale.

Linking structure to fragility in bulk metallic glass-forming liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk metallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the supercooled liquid near Tg. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neighbor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure.

Phase decomposition and nano structure evolution of metastable nanocrystalline Cu-Co solid solutions during thermal treatment

Nanocrystalline and ultrafine-grained Cu100-xCox (x = 26 and 76) solid solutions have been prepared by severe plastic deformation (SPD) of elemental powder mixtures. For both concentrations a supersaturated solid solution fcc phase was identified after the deformation process with grain sizes of less than 50 nm for Co rich solutions and around 100 nm for Cu rich solutions. Additionally, synthesis of nanocrystalline materials in the Cu-Co alloy system by electrodeposition has been conducted. Microstructural characterization by scanning and transmission electron microscopy, differential scanning calorimetry, and microhardness measurements are used to investigate the structural evolution, the thermal stability and mechanical properties of the different nanocrystalline Cu-Co alloy materials during isothermal and non-isothermal annealing. In this study it is shown that the phase decomposition of the metastable Cu-Co solid solutions has a significant influence on their thermal stability, which can be linked to the underlying microstructure that forms during annealing.